In recent years, Cu(In,Ga)Se2 (CIGS) materials have been extensively studied for use as an absorber layer in thin film photovoltaic devices, owing to their band gaps that can be tuned by adjusting the elemental ratios and are well matched with the solar spectrum (1.0 eV for CuInSe2 to 1.7 eV for CuGaSe2), offering potentially high conversion efficiencies; 20.3% conversion efficiency was achieved using Cu(InxGa1-x)Se2 material by researchers at ZSW and the Centre for Solar Energy and Hydrogen Research in Germany (August 2010). One drawback of CIGS materials is the high manufacturing cost, due to the high cost of the constituent elements.
Cu2ZnSnS4 (CZTS) materials can be used as a low-cost alternative to traditional Cu(In,Ga)Se2, due to the abundance and low toxicity of Zn and Sn, which are much cheaper than Ga and the rarer In. CZTS is reported to have a band gap between 1.45 and 1.6 eV [H. Katagiri et al., Appl. Phys. Express, 2008, 1, 041201; K. Ito et al., Jpn. J. Appl. Phys., 1988, 27 (Part 1), 2094; T. M. Friedlmeier et al., Proc. 14th European PVSEC, Barcelona, Spain, 30 Jun. 1997, p. 1242] and a high optical absorption coefficient (up to 105cm−1) [G. S. Babu et al., J. Phys. D: Appl. Phys., 2008, 41, 205305], which are similar to those of CuInGaSe2.The current record conversion efficiency for pure Cu2ZnSnS4 of 8.4% [B. Shin et al., Prog. Photovolt.: Res. Appl., 2013, 21, 72] shows great potential for this material.
The highest efficiency CZTS and CIGS solar cells are fabricated using a vacuum-based deposition method. Vacuum-based approaches typically offer high uniformity, which translates to a high quality film. However, the techniques are also generally costly, with material consumption and energy usage being high. Non-vacuum-based approaches are attractive in that they are typically higher throughput processes, with a lower deposition cost. One such method is a nanoparticle-based deposition approach. Nanoparticles of CZTS material can be fabricated, then subsequently processed into an ink or slurry, which can be printed onto a substrate using low-cost printing techniques, such as spin-coating, slit-coating, doctor blading, inkjet printing, and the like. The films are then sintered at elevated temperatures to induce growth of larger crystal grains within the film, which are necessary to achieve high power conversion efficiencies since recombination of charge carriers can occur at grain boundaries. Nanoparticles are advantageous, since they have a reduced melting point relative to the bulk material, facilitating lower temperature device processing; the smaller the nanoparticles, the lower the melting point. A uniform particle size distribution is also favourable, since particles of the same size will melt uniformly.
Nanoparticles can be synthesised from a top-down or a bottom-up approach. In a top-down approach, macroparticles are processed, e.g. using milling techniques, to form nanoparticles; the particles are typically insoluble, therefore difficult to process, and in the case of milling the size distribution may be large. Using a bottom-up approach, whereby nanoparticles are grown atom-by-atom, smaller particles with a homogeneous size distribution may be produced. Colloidal syntheses can be employed to grow nanoparticles in solution, which can be surface passivated with organic ligands to provide solubility, and thus solution processability.
The colloidal methods of making CZTS nanoparticle materials described in the prior art have one or more disadvantages. For example, the methods use hot-injection, high boiling capping agents, long reaction times, and/or unfavourable reagents for commercial processes, and/or impurity phases may be formed.
Hot-injection techniques can be used to synthesise small nanoparticles with a uniform size distribution. The technique relies on the injection of small volumes of precursors into a large volume of solvent at elevated temperature. The high temperature causes breakdown of the precursors, initiating nucleation of the nanoparticles. However, the technique results in low reaction yields per volume of solvent, thus making the reactions difficult to scale to commercial volumes.
Other prior art techniques utilise high boiling ligands, such as oleylamine. Organic ligands assist in solubilising the nanoparticles to facilitate solution processability, yet they must be removed, e.g. by evaporation, prior to sintering, since residual carbon can be detrimental to the performance of the absorber layer. Thus it is favourable that the boiling temperature of any capping ligand(s) should be substantially lower than the sintering temperature of the CZTS film.
Relatively short reaction times to produce the nanoparticles are advantageous, to minimise the total processing time from nanoparticle synthesis to functioning PV device.
In the prior art, when the CZTS nanoparticle synthesis takes place at temperatures below 180° C., an impurity phase has been observed. [T. Kameyama et al., J. Mater. Chem., 2010, 20, 5319] Phase purity is preferable, in order to achieve a uniform sintering temperature and thus achieve a high quality film.
Some of the methods in the prior art utilise precursors that are unfavourable for commercial production. For example, Liu et al. describe the synthesis of (ZnS)x(Cu2SnS3)1-x nanocrystals [Q. Liu et al., Chem. Comm., 2011, 47, 964] using a dibutyldithiocarbamic acid precursor solution that is prepared using carbon disulphide. Carbon disulphide is not only highly flammable, but can also affect fertility and cause organ damage following repeated exposure; these factors have implications on the scalability of the synthesis.
Thus, there is a need for a commercially scalable synthesis of CZTS nanoparticles, utilising reagents that can be handled safely on an industrial scale, with a relatively low boiling capping agent that is suitable for lower temperature PV device processing.